Astrophile: Undead stars rise again as supernovae

Astrophile is our weekly column covering curious cosmic objects, from within the solar system to the furthest reaches of the multiverse

Object type: white dwarfNumber known: 10 in our galaxy

Ten thousand light years away, the burned-out core of a dead star quietly circles a sun-like companion. Though the stellar corpse shows no signs of life, it is a cosmic vampire, biding its time as it slowly sucks gas from its mate.

Decades later, a blinding flash 100,000 times brighter than the sun heralds the undead star's reawakening: it has finally accumulated enough stolen fuel to power nuclear fusion once more. The star shines brightly for a few glorious days before returning to its deathlike slumber for years or decades, until the whole sequence repeats itself.

Spectacular as they are, these resurrections are just the prelude to the final act, when the undead star will go supernova, finally obliterating itself as it outshines our entire galaxy.

That at least is the suggestion of recent measurements of one such sleeper star, also known as a recurrent nova. They support the theory that these novae are the long-sought progenitors of a very interesting kind of exploding star: type 1a supernova.

Nature of darkness

Finding these progenitors would be a boon to the study of dark energy, the mysterious entity thought to be accelerating the expansion of the universe. It was type 1a supernovae that led to the identification of the mysterious stuff in the first place, garnering three cosmologists a Nobel prize earlier this year. All type 1a evolve from a type of star called a white dwarf, but pinning down exactly which white dwarfs are supernova precursors could lead to much more precise measurements of dark energy – and even reveal its true nature.

The hunt has been on for decades. Recurrent novae were first discovered in 1913, but it wasn't until the 1970s that they became prime suspects. That was when they were identified as heavy white dwarfs with a mass very close to the supernova "tipping point" of 1.4 times the mass of the sun. When a white dwarf grows heavier than this, it can no longer support its own weight and starts collapsing, triggering nuclear reactions that rip the star to shreds in a type 1a supernova.

Still it has been difficult to prove that recurrent novae get massive enough to make the transition from heavy white dwarf to type 1a explosion. They steal gas from their neighbours, but also shed it during their outbursts, so it wasn't clear whether they gain or lose material overall.

Gain or lose?

To settle this question, Bradley Schaefer of Louisiana State University in Baton Rouge analysed measurements of the recurrent nova CI Aquilae from before and after its 2000 eruption.

Heavier pairs of stars orbit each other faster because of their stronger gravity. That means that any mass lost by the white dwarf would lengthen its orbital period.

Schaefer's team found that there was no measurable change in CI Aquilae's 15-hour orbital period after the eruption. Given the accuracy of their observations, this means the white dwarf cannot have lost more than one-millionth of the sun's mass in the event.

As it is estimated to steal more than twice that amount from its companion between eruptions, it must gain mass overall, Schaefer concludes.

Eagle-eyed amateurs

The conclusion is tentative because of possible measurement errors. But fortunately, eagle-eyed amateurs have caught two more of the 10 known recurrent novae in the process of erupting – U Scorpii in January 2010 and T Pyxidis last April.

T Pyxidis was a surprise, but Schaefer had predicted when U Scorpii would rise again, so space telescopes and ground-based observatories were ready to pounce on it. "We plastered that thing with observations – it was awesome," Schaefer says.

The analysis of those observations, along with measurements of orbital periods over the next few years, could help recurrent novae beat rival candidates for the role of true type 1a progenitors.

That would be a breakthrough for the study of dark energy. Type 1a supernova all seem to have the same intrinsic brightness, so their apparent brightness can be used to work out how far away they are. That, in turn, allows us to estimate how fast the universe's expansion is accelerating. However, these so-called "standard candles" do vary slightly from one another, limiting the precision of such measurements.

Knowing the properties of the stars that produce these type 1a explosions could help researchers better understand their variations, allowing more precise estimates of the acceleration of cosmic expansion. These in turn will be crucial to distinguishing between different theories for dark energy's origin.

"You can't get that high accuracy unless you know what the progenitor is," says Schaefer. "We desperately need to know."

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